The present invention relates to methods for facilitating the transfer of nucleic acids into cells and to a novel cationic amphiphile useful for this purpose.
Some but not all cationic amphiphiles are known to facilitate the transfer of DNA into cells, i.e., transfection. Although the mechanism of this activity is not yet clear, it probably involves the binding of the DNA/lipid complex with the cell surface via the excess positive charges on the complex. Cell surface bound complex is probably internalized and the DNA is released into the cytoplasm of the cell from an endocytic compartment. How the released DNA moves into the nucleus is not known.
A cationic amphiphile contains the following four important structural elements:
lipophilic ------- Linker ------ Spacer ------ Amino group bond arm group
The amino group is positively charged at neutral pH. It may be a primary, secondary, tertiary or quaternary ammonium group. The spacer arm is usually a hydrophilic, 2 to 15-atom moiety which connects the amino group to the lipophilic group via the linker bond. The linker bond is either an ether, ester, amide or other hydrolyzable bond.
The lipophilic group is a hydrophobic moiety which allows the insertion of the cationic amphiphile into the membranes of the cell or liposome. It serves as an anchor for the cationic ammonium group to attach to the surface of a cell or liposome.
N-[1-(2,3-dioleoxyloxy) propyl]-N,N,N-trimethyl ammonium chloride (DOTMA) is the first cationic amphiphile exhibiting the activity of transfection. Its lipophilic group is a double-chain, C18:1 aliphatic group. It contains a quaternary ammonium group connected to the lipophilic group via a 3-carbon spacer arm with two ether linker bonds. Although the molecule is effective in transfection, it is not biodegradable and is rather toxic to cells.
Another series of cationic amphiphiles used in transfection is the quaternary ammonium detergents. Either single chain (such as cetyltrimethylammonium bromide) or double chain (such as dimethyldioctadecylammonium bromide) detergents exhibit activity to transfect animal cells. The amino group in these amphiphiles is quaternary and is connected to the lipophilic group without the spacer arm or linker bonds. Another single-chain detergent, stearylamine, contains a primary amino group connected to a single C18:0 chain without a spacer arm or linker bond. This group of amphiphiles is also toxic to the cells.
Two other groups of cationic amphiphiles for transfection have been reported. The first group contains two C18:1 chains as the lipophilic group. The second group contains a cholesterol moiety as the lipophilic group. Both groups contain a quaternary ammonium group, but the spacer arm structure varies. In one case, the trimethylammonium group is directly connected to the two C18:1 chains via a 3-carbon spacer arm and ester bond. The amphiphile, 1,2-dioleoxy-3-(trimethylammonio)propane, (DOTAP) is a close analog of DOTMA. In other cases, such as 1,2-dioleoyl-3-(4'-trimethylammonio) butanoyl-sn-glycerol, DOBT, or cholesteryl (4'-trimethylammonio) butonate, ChOTB, the trimethylammonium group is connected via a butanoyl spacer arm to either the double-chain (for DOTB) or cholesteryl (for ChOTB) group. Other amphiphiles, i.e., 1,2-dioleoyl-3-succinyl-sn-glycerol choline ester (DOSC) and cholesteryl hemisuccinate choline ester, ChOSC, contain a choline moiety as the quaternary ammonium group which is connected to the double-chain (for DOSC) or cholesteryl (for ChOSC) group via a succinyl spacer arm. The transfection activities of these amphiphiles are generally weak.
Yet another class of amphiphiles, called "lipopolyamine" has also been reported. The ammonium group is L-5-carboxyspermine which contains 2 primary and 2 secondary ammonium groups. Two examples of this lipopolyamine are dioctadecylamidologlycylspermine, DOGS, and dipalmitoyl phosphatidylethanolamidospermine, DPPES. The cationic group is connected to two different double-chain, C16:0 lipophilic group via an amidoglycyl (for DOGS) or phosphorylethanolamine (for DPPES) spacer arm. These compounds are especially efficient in transfecting the primary endocrine cells without cellular toxicity.
A lipopolylysine reagent for transfection has also been reported. The reagent contains a polylysine moiety as the ammonium group which is connected to a phospholipid (N-glutarylphosphatidylethanolamine). Therefore, the spacer arm is the side chain of lysine and the head group of the phospholipid. The lipophilic group is a double-chain, C18:1 group connecting to the spacer arm via two ester bonds. Although the reagent is efficient in transfection and non-toxic to cells, the activity requires scraping the treated cells. This is clearly not a convenient step and cannot be done for in vivo experiments.
An ideal transfection reagent should exhibit a high level of transfection activity without scraping or any other mechanical or physical manipulations of the cells or tissues. The reagent should be non-toxic or minimally toxic at the effective doses. It should also be biodegradable to avoid any long-term adverse side-effects on the treated cells.
Many reagents which fulfill these criteria contain a linker bond that is hydrolyzable in the cell. For example, DOBT and DOSC, both contain ester linker bonds, can be metabolized and catabolized into other lipid species in the treated cells. However, cationic amphiphiles containing ester linker bonds are not stable when stored in an aqueous solution. This is probably due to a base-catalyzed hydrolysis reaction mediated by the amino group of the amphiphile.
Another key factor on the cellular toxicity of the cationic amphiphiles is their inhibitory effects on the activity of protein kinase C (PKC). PKC is a key enzyme which plays a crucial role in cellular signal transduction. Cationic amphiphiles inhibit PKC activity by mimicking the endogenous inhibitor, sphingosine. PKC activity is also important for the cellular endocytosis pathway which is likely to be involved in the action of the cationic amphiphiles to facilitate the entry of DNA into cells. Recently it has been reported that a PKC activator, phorbolmyristateacetate, can stimulate the transfection efficiency of DNA mediated by the calcium phosphate precipitates.